when should I use engine room blower

My trawler has a blower in the engine room; something I never had on the sailboat. Both the main engines and the generator are diesel. Is the blower simply to cool off the engine room after running on a hot summer afternoon, or is there more to it than that?

Typically I would use mine while the engines are running it helps to provide 02 for the engine and removes some of the warm fumes and smells that the hot engine may create.

seattleboatguy said:

My trawler has a blower in the engine room; something I never had on the sailboat. Both the main engines and the generator are diesel. Is the blower simply to cool off the engine room after running on a hot summer afternoon, or is there more to it than that?

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Is it an intake or exhaust blower?

Northern Spy said:

Is it an intake or exhaust blower?

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Interesting question. I assumed it was an exhaust blower, but I suppose it is at least possible it is an intake blower. There are small stainless vents on the hull, so I would guess that the vents take care of the intake part, and if so, the blower would probably be for exhaust. But, I'll do an experiment tonight after work to be sure.

seattleboatguy said:

My trawler has a blower in the engine room; something I never had on the sailboat. Both the main engines and the generator are diesel. Is the blower simply to cool off the engine room after running on a hot summer afternoon, or is there more to it than that?

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My diesel has one (exhaust). It won't have any affect on ER temps etc as flow is small compared to intake air volume. Intake air provides significant vol of fresh air to purge ER. and most diesels vent crankcase near intake which should remove oil vapor & odors.
I do use mine as you suggest... Helps cool ER after shut down.

Don
'08 MS34HT
"Bacchus"

Operating in SE Alaska I have never found the need to run my engine room exhaust fan while under way. I have used it to cool engine room if I need to be in it after cruising and to get rid of steam when my old genset has over heated. When I bought the boat, I discovered that the squirrel cage type fan-which was mounted at the rear of the engine room-was wired for blowing rather than exhausting air. This did not make sense, so I reversed the fan.

Tator

There are two types of air requirements in your Engine Room. One is "cooling air" which is supplied to remove radiant heat and fumes to make the ER more comfortable, assist in corrosion inhibition, etc.. The second and what I consider a major item is "combustion" air. That is air necessary for the proper combustion in the engines. It is common that most engine will use about 2.5 cubic feet of air per minute per HP. So a 300 HP engine will require about 750 cubic feet of air per minute. That is why most engine room blowers blow into the ER, why make the engines work harder to get the air they need ? Your marine engine owner's manual should have a chapter on this item.

Blowers are normally used to cool the engine room after running. Check your blower's rating as many are not rated for constant duty service.

Monitor your ER temp when underway, as some ER's do need help to get all the air the engines need.
A quick test is to open the door or lift the hatch when underway, if you feel negative pressure it will indicate a need for more air, as well as black transom.
50 square inches per 100 hp is a good rule of thumb for open area needed to supply adequate air, more is better if possible.

It's pretty simple, there is no need to run exhausted blowers while underway as they just compete with the engine/s for the ambient incoming air to the engine space. Run them after running to pull out hot air and fumes if need be. Or of course before starting gas engines.

Running intake blowers while underway can help feed the engine cool air while running.

Capt.Bill11: Is correct if your blowers are exhausting the air from the ER do not run them while the engines are running unless you have a system that also supplies twice as much intake air. The air temp he alluded to (cool air) is also of critical importance. Cool air from you intake air blower will allow the engine to run more efficiently.

Just thinking out loud here, but do you think those little gerbil cage blowers could, in any way, take a measurable amount of air away from an air-gulping thing like a diesel motor? I would kinda doubt it, but then again, MY engine room has two 6-litre turbo charge monsters in there that would suck in a phonebook at WOT. Still, even a single Perkins sucks air at a pretty good clip. I would wonder if a simple blower could make any difference at all. No data... Just a curious question.

Tom.B said:

Just thinking out loud here, but do you think those little gerbil cage blowers could, in any way, take a measurable amount of air away from an air-gulping thing like a diesel motor? I would kinda doubt it, but then again, MY engine room has two 6-litre turbo charge monsters in there that would suck in a phonebook at WOT. Still, even a single Perkins sucks air at a pretty good clip. I would wonder if a simple blower could make any difference at all. No data... Just a curious question.

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If they are the little cheap junk ones you might be right. But they would still be an unnecessary restriction.

The better ones move 250 cfm or so. So a pair of them are trying to pull out 500 cfm.

Ours is an exhaust blower and I generally run it while underway for two reasons. The first is an assumption, but with twin opening portlights port and starboard I typically have those fully open (unless sea conditions warrant closing them) for fresh air/combustion intake. So I'm assuming that with the blower located near the ER overhead, I am mechanically cycling out air which aids in drawing in fresh air. Second reason is it keeps the "engine smell" at bay.

Of course, I run the blower when using the generator for the same reasons, especially as we are usually at anchor and not getting the same air movement we get while underway. Also aids in generator smell abatement.

My wife has a bionic olfactory system...

Here's how to figure out the CFM air flow an engine needs.


Air Flow Calculation Formula

Metric: Air flow in m3/min = swept volume1 (liters) x speed2 x VE3 x PF4 1000 x CF
1 engine displacement in liters; 2 maximum engine RPM; 3 Volumetric Efficiency; 4 Pulsation factor

Imperial: Air flow in CFM = swept volume5 (CID) x speed2 x VE3 x PF4
1728 x CF
5 engine displacement in cubic inches; 2 maximum engine RPM; 3 Volumetric Efficiency; 4 Pulsation factor


Volumetric Efficiency (VE)
- VE = can be greater than >2 for very new engine designs
- VE = 1.3 to 1.8 for 4 stroke engine with turbocharger
- VE = 0.85 for 4 stroke engine that is naturally aspirated
- VE = 1.4 for 2 stroke (cycle) engine with Roots-Compressor (blower) - VE = 1.9 for 2 stroke (cycle) engine with Turbocharger
Cycle Factor (CF)
- CF = 2 for a four stroke (cycle) engine
- CF = 1 for a 2 stroke (cycle) engine
Pulsation Factor (PF) - only applies to engines that are both naturally aspirated and having 3 cylinders or less - PF = 2-2.1 if only 1 cylinder
- PF = 1.4-1 for two cylinders
- PF = 1.33 for three cylinders

The engine itself creates lots of air circulation while underway, sucking air. I don't use the exhaust fan except if entering the engine compartment to reduce my discomfort. Can't imagine an exhaust fan significantly increasing engine-cooling rate. A gasoline engine, on the other hand, is subject to explosive fumes, and the fan should be operated a few minutes before engine starting.

In one minute an engine will use and exhaust roughly the same amount of air as would be found in a small engine room (14 x 12 x 5). If it is not provided a source of air it will create it own by pulling it in from vent openings, hatchways, port holes, anyway it can. As many have said before, nature abhors a vacum. It will even pull air through broken electrical conduits, old piping, and small cracks in your bulkheads many places you probably do not want salt ladden air moving.

For those of you that do not think supplying more than ample air to you engine is a critically important issue, I would assume all of your engines are naturally aspirated and no turbos are used.

My IG has an OEM exhaust blower located at an ER ventilator to port, and an identical ventilator without mechanical assistance to stbd. Its switch is one of 3 identical switches located together on the engine start panel, the other 2 are the engine start switches, there`s clear hint, I`d say.
The IG Operators Manual says: "The primary function of the blowers is to control engine room air temperature".
So I run the blower when I run the engines. The couple of times I forgot it got seriously hotter in the ER than with blower operating.

For intake assistance I have a pair of Delta T's rated at 737 cfm each. They are for the front ER vents, 4 in total each 230 sq in opening. Then at the rear of the ER I have a pair of 4" squirrel cage fans, 350 cfm each, setup as exhaust fans to the rear ER vents (2 x 230 sq in). To keep the ER a bit cooler I run all four fans when the engine is operating. Sometimes I run the exhaust blowers after shutdown but I don't think it makes much difference as the ER stays quite warm for a long time after shutdown.

I would use them when Temps are high in er. All the stuff in thier runs better last longer if not hot

Intake (shown here) and exhaust vent (on the opposite side) of the engine compartment are of the same size. Note the intake grill at the lower aft end of the pilothouse in this photo:

"I assumed it was an exhaust blower, but I suppose it is at least possible it is an intake blower."

On our sized boats exhaust is most common.

The reason is most ER are not well sealed from the living spaces and most folks do not want the ER stink/noise of hot engine(s) pushed into the cabins.

The engine intake system hopefully is designed to feed air at max throttle , since many cruise well below that a tiny DC fan is meaningless.

Use it to lower ER temps after shutdown if desired.

Northern Spy said:

Is it an intake or exhaust blower?

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I tested it this morning, and much to my surprise, it was an intake blower. But, after reading the posts from this thread, I can understand the reasoning for installing an intake blower rather than an exhaust blower. And, I would guess that even with an intake blower, it could still be used to cool off the engine room at the end of a day's run, since there are air vents built into the hull for engine room ventilation.

My sporty has eight 10 inch fans, 4 intake and 4 exhaust with an adjustable temp switch. Basically they just move a lot of air thru the engine area. 10,000 lbs of 180 degree iron is a pretty good load for the aircon system to handle. The fans help with that. They dont usually come on while underway, so I assume the engines are moving enough air on there own.

In FL exhaust blowers help cool the ER after a run. In my boat it helped the cabin AC a little. I wished there was room for a large exhaust fan.


Air will flow in through turned off exhaust fans if the ER becomes lower pressure than outside.


Big diesels running at high power levels consume a large amount of air.

"Big diesels running at high power levels consume a large amount of air."

However the fuel to generate the the amount of DC (10A?) will probably slow the engine down more than the high altitude a 10A fan might produce.

seattleboatguy said:

I tested it this morning, and much to my surprise, it was an intake blower. But, after reading the posts from this thread, I can understand the reasoning for installing an intake blower rather than an exhaust blower. And, I would guess that even with an intake blower, it could still be used to cool off the engine room at the end of a day's run, since there are air vents built into the hull for engine room ventilation.

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Or it's just going to push hot air and fumes into the boat.

I am not sure how much "altitude" increase a fan will produce but you can achieve 200 or more cf per minute with about 1 amp ac 120v .25 hp blower.

FF said:

"Big diesels running at high power levels consume a large amount of air."

However the fuel to generate the the amount of DC (10A?) will probably slow the engine down more than the high altitude a 10A fan might produce.

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I don't think you could measure either.

The other fear of an intake blower is spreading a fire from the ER to the vessel.